Plyr mapvalues to rename levels giving unexpected result

Question

I am trying to use mapvalues to rename a long list of levels of a factor. As it seemed not to be working correctly, I produced a simple example showing the issue.

x <- c("a", "b", "c","c","d","a")
y <- c("a", "c", "c","d","d","b")
z<-data.frame(cbind(x,y))

levels(z$y)<-mapvalues(z$y, c("a", "c","b"), c("A", "C","m"))
levels(z$x)<-mapvalues(z$x, c("a", "c","b"), c("A", "C","m"))

Answer:

>z
x y
1 A A
2 m C
3 C C
4 C d
5 C d
6 A C

As can be seen, $x[5] should be "d" and $y[6] should be "m". I also tried applying "as.character" to "from" "to" vectors but it didn't work either. I'm working in RStudio, Mac version, R version 3.5.2 (2018-12-20) -- "Eggshell Igloo". Thanks for help.

Answer 1

If we look at the output of

plyr::mapvalues(z$y, c("a", "c","b"), c("A", "C","m"))
#[1] A C C d d m
#Levels: A m C d

It is a vector with the same length as 'x', while the levels length is different

If we assign this output to the levels(z$y )`, there is length difference. We can either do the assignment to the whole column

z$y <- plyr::mapvalues(z$y, c("a", "c","b"), c("A", "C","m"))

Or change the levels with the corresponding levels

levels(z$y) <- levels(plyr::mapvalues(z$y, c("a", "c","b"), c("A", "C","m")))